findHomography, getPerspectiveTransform, & getAffineTransform

Question

This question is on the OpenCV functions findHomography, getPerspectiveTransform & getAffineTransform

1. What is the difference between findHomography and getPerspectiveTransform?. My understanding from the documentation is that getPerspectiveTransform computes the transform using 4 correspondences (which is the minimum required to compute a homography/perspective transform) where as findHomography computes the transform even if you provide more than 4 correspondencies (presumably using something like a least squares method?). Is this correct? (In which case the only reason OpenCV still continues to support getPerspectiveTransform should be legacy? )

2. My next concern is that I want to know if there is an equivalent to findHomography for computing an Affine transformation? i.e. a function which uses a least squares or an equivalent robust method to compute and affine transformation. According to the documentation getAffineTransform takes in only 3 correspondences (which is the min required to compute an affine transform).

Best,

Answer 1

Q #1: Right, the findHomography tries to find the best transform between two sets of points. It uses something smarter than least squares, called RANSAC, which has the ability to reject outliers - if at least 50% + 1 of your data points are OK, RANSAC will do its best to find them, and build a reliable transform.

The getPerspectiveTransform has a lot of useful reasons to stay - it is the base for findHomography, and it is useful in many situations where you only have 4 points, and you know they are the correct ones. The findHomography is usually used with sets of points detected automatically - you can find many of them, but with low confidence. getPerspectiveTransform is good when you kn ow for sure 4 corners - like manual marking, or automatic detection of a rectangle.

Q #2 There is no equivalent for affine transforms. You can use findHomography, because affine transforms are a subset of homographies.